The claimed inventions relate generally to systems that control line pressure in an automatic transmission by a modulation pressure, and more particularly to transmission modification kits that provide line pressure control as a substitute or supplement for computer-controlled line pressure through a modulation pressure circuit and solenoid.
For orientation, the operation of automatic transmissions will now be described. Shown in FIG. 1 are conceptual elements of operation of most automatic transmissions. A pump 1 is configured to receive fluid, typically oil, from a reservoir or pan 4 passing through a filter 3. Pump 1 is driven by rotation of an input shaft coupled to an engine, not shown. In most transmissions a positive displacement pump is used, which is a pump that generates a substantially constant volume per input shaft rotation, or a fluid flow substantially proportional to the rotation of the input shaft. Examples of positive displacement pumps are georotor, gear and vane type pumps. While the input shaft is being driven by rotation of the engine, pump 1 produces a flow of fluid that returns to the reservoir 4 except for a small portion needed for lubrication and to displace valves and servos. Fluid pressure is produced by a main regulator 2 by partially blocking the flow produced by pump 1, which “line” pressure is distributed throughout the transmission through passages 5.
The line pressure is typically provided at a pressure higher than needed by the operating transmission components, and is regulated down to lower operating pressures by auxiliary regulators 6a and 6b. Those lower pressures are provided to the transmission components, in this example valves 7a and 7b and servos 8a and 8b. The arrangement shown in FIG. 1 is merely conceptual; in practice many different configurations of regulators, valves and servos are be used, as is understood by one of ordinary skill in the art.
Fluid pressure may be applied to the several servos in the transmission to provide mechanical operation of the driven components. Those components ordinarily include several clutches and a torque converter, by which the several gears of the transmission are applied to the engine output. An accumulator is normally coupled to the input of a servo, which slows the engagement or disengagement of the servo. Although an accumulator might be implemented by a spring and piston in a bore, conceptually one operates as a balloon. If pressure is increased, an accumulator accepts fluid until an equalibrium is reached. Likewise, as pressure decreases, the accumulator discharges fluid to the new set pressure. The movement of fluid in or out of an accumulator is not instantaneous, but rather is slowed by the fluid passages of the transmission. An accumulator thereby functions to buffer input pressures and graduate the transitions of servo engagement and disengagement.
The gradual operation of servos tends to soften the shifts of the transmission. Sudden gear transitions are undesirable, because passengers feel a lurch or impact and because undue stress is applied to the engine and drive components. Gear shifts that are too soft, however, are also undesirable. During the transition from one gear to another, two clutches may be engaged for a time which increases wear and heat in the transmission. Soft shifts increase this transition time, which decreases the service life of the transmission. A great deal of research and design effort has been made to optimize the shifts in transmissions to balance this tradeoff.
It has been recognized that firm shifts are preferable in some driving circumstances, such as during hard acceleration. Soft shifts, on the other hand, are preferable under other circumstances, for example under light acceleration and coasting. One method of acheiving both hard and soft shifts in the same transmission is to vary the pressure applied to the servos and accumulators. A lower engagement pressure to a servo results in an increased transition time, as more time is required to “fill up” the accumulator. Likewise, a higher disengagement pressure may also be helpful to soften a shift.
One technique used to adjust fluid pressure to servos is through controlling line pressure. A higher line pressure will cause faster servo transition, at least to engagement. As a servo is to be engaged, its accumulator must first accommodate the new pressure. It does so by accepting an amount of fluid which the system must supply through the line pressure. This flow must pass through the various restrictions in the transmission passages, and can do so more rapidly if the head pressure is higher. Thus a higher line pressure will force a greater fluid flow through the transmission passages, which accordingly causes more rapid accumulator adjustment and firmer shifts. This technique also applies to the movement of valves, which also requires some amount of fluid to enter a chamber at the end of the valve bore.
Modulators capable of adjusting fluid pressure have included throttle valves with mechanical linkage and vacuum modulators. These have worked to increase transmission fluid pressure when the throttle is open, intending to cause firmer shifts under that condition. Most recently, modulators have been coupled to an automotive computer/controller that controls the transmission line pressure. Modem automobiles feed a number of sensor inputs into a computer, which then operates to control any number of operational parameters, such as the timing of fuel injectors and spark plug ignition timing. The computer is carefully designed to provide good performance, especially under average driving conditions.
Referring now to FIG. 2A, a line pressure modulation system is shown capable of being controlled by an automotive computer. As in the system of FIG. 1, a pump draws fluid through a filter 3 and supplies fluid flow to a main regulator 5, which provides regulated line pressure 5. Main regulator 2 includes a modulation port by which the line pressure may be controlled, for example, in a modulation circuit at a pressure proportional to the modulation pressure in a given operating range. Restrictions 14 and 15 provide pressure isolation between the modulation port and the input port of the main regulator 2. Restrictions 11 and 12 are conceptual in nature; in practice these restrictions might be provided by passages in the transmission, by regulators, or by other components that supply isolation between the two circuits. A solenoid or modulator 9 is coupled to the modulation pressure 13 providing relief whereby the modulation pressure may be controlled or regulated. A pressure relief valve 10 including a fluid exhaust port is provided to vent a damagingly high modulation pressure 13, which may also limit the maximum line pressure 5 that can be developed in the system. When solenoid 9 in the example is inactive, no fluid flow occurs through the modulation passages under normal conditions. Solenoid 9 may be fully driven to acheive a low modulation pressure 13, or may be partially driven to acheive a moderate modulation pressures through pulse-width modulation techniques, for example by an automotive computer.
The configuration shown in FIG. 2 has two inherent failure conditions. First, if the solenoid should become disconnected from the computer, or if the solenoid became stuck “off,” the modulation pressure will rise to its maximum. This failure will result in hard shifts at all times, and may result in damage to transmission components, such as the pump, if line pressure is excessive. In some transmissions cooler and lubrication flow may be reduced or shut-off with excessive modulation pressure, as will be discussed below. Second, if the solenoid should become stuck “on”, the modulation pressure will stay low, resulting in soft slippery shifts at all times. This may cause overheating and failure of the transmission, especially for vehicles towing loads up grades.
That configuration has a third failure mode, which is failure of the computer to appropriately command line pressure. The designer of the system may have considered only limited circumstances of use, and designed the computer's program for only the “normal” operational use. For example, it is not uncommon for a single transmission model to be installed to both standard passenger and towing vehicles, despite the large potential difference in total weight. The transmission design may be optimized for a passenger car or a medium duty truck, and may be found to perform acceptably well in the heavy-duty towing vehicle such that an additional transmission model is not necessary to develop or maintain an assembly line for. Under actual use that transmission might be subjected to heavier loads than what the designer intended, because, for example, an operator finds the vehicle engine is sufficiently powered to tow a load up a certain grade. The vehicle's computer may not have a sensory input for the tow weight, and may command soft shifts where firm shifts are called for to avoid transmission overheating. Additionally, most automotive companies do not provide for any automotive computer reprogramming as a solution.
A fourth failure mode may be encountered with the failure of an engine or transmission sensor. It is not unknown for vehicle owners or drivers to continue to operate a vehicle even though the check-engine light is on, indicating that an automotive computer has discovered a problem and recorded a trouble code. Indeed, a vehicle operator may be unmotivated to have the vehicle diagnosed and repaired, due to an expected high cost. Furthermore, some older vehicles were designed only to check for electrical continuity of sensors, and not to detect and flag out-of-range conditions caused by failing sensors. A failed sensor may cause incorrect control of a transmission. For example, a faulty throttle position sensor may cause an automotive computer to erroneously recognize a full-throttle condition as a mid or low-throttle condition. The computer might then command low line pressure for softer shifts, increasing heat and wear. Many other undesirable effects may occur from the failure of other vehicle sensors.
Automotive systems, and especially transmission systems, are operationally complex and require a great deal of knowledge and experience to diagnose and repair problems not frequently encountered. Problems with line pressure are not always perceptible with a vehicle “in the shop,” particularly if those problems occur only under special circumstances, for example towing a heavy load up a long and steep grade. Furthermore, it is uncommon for a mechanic or driver to observe transmission line pressure out of the shop because of the difficulty installing a gauge that can be seen from the safety of the inside of a moving vehicle, which might be the only way to directly observe certain transmission performance problems. Trained but inexperienced mechanics may follow the standard flowcharts and/or instructions and observe proper performance under normal conditions, but fail to understand the nature of a particular transmission failure. Furthermore, there has been deficit of understanding of the operational relationship between an automotive computer and a transmission in recent automobiles, which has allowed many transmission problems to continue without a solution for several years. Indeed, there has been a need for a way to provide reliable modulation pressure in a transmission independently of an automotive computer for some time.